This application describes a proposal to develop a comprehensive family registry of individuals who are at high risk for breast cancer. The registry will consist primarily of individuals ascertained from Utah and the surrounding region, but will incorporate information and biological samples from family members regardless of location of residence. The registry will draw upon the strengths of this population and environment: the presence of large families with well-defined pedigrees (i.e. large, informative families); the recently established IRB-approved High Risk Breast Cancer Clinic (HRBCC); and the existence of well-known programs in genetics and genetic epidemiology. The establishment of such a registry will provide the resources for a wide variety of studies on the etiology, prevention, and treatment of breast cancer. Specifically we propose to: 1. Create a Family Registry which includes: a. Families of individuals who are identified as genetically predisposed to breast cancer. b. Detailed epidemiologic and genetic data on family members. c. Biological specimens. 2. Provide counseling and follow-up on the individuals enrolled in the Clinic. 3. To conduct a pilot study examining the radiation sensitivity of breast epithelial cells from women with BRCA1 mutations, and testing the hypothesis that BRCA1 carriers demonstrate genomic instability and accelerated carcinogenesis.